1. Field of the Invention
The present invention relates to a cold spray apparatus having a powder preheating device. In more particularly, the present invention relates to a cold spray apparatus having a powder preheating device, capable of obtaining high deposition rate and excellent coating layer under the same spray processing conditions by preheating coating powder before a coating process.
2. Prior Art
A thermal spray coating method is widely used to coat material to a substrate. In the thermal spray coating method, a substrate that is a parent material is roughened by a blasting process and is coated by a mechanical bonding. That is, a powder is melted by several heat sources, such as electric arc and plasma, and then is sprayed on the substrate at high velocity. In this manner, the powder is coated on the substrate.
Such a thermal spray coating method can coat almost all kinds of material. Also, substrate temperature is increased slightly and a relatively thick coating is possible at a short time. For these reasons, the thermal spray coating method has been widely used in many industrial fields.
However, an original structure of the coating powder may be changed due to the melting of the coating material. Specifically, in the case of special structures like a nano or amorphous structure, material is melted even if raw material has nano or amorphous structure. Thus, after the coating, the resultant structure can hardly hold the original nano or amorphous structure.
When a material such as nano-structured WC—Co is sprayed at high velocity, a large area of the powder is exposed to the heat source and therefore WC is easily decomposed into a vulnerable carbide such as W2C, W3Co3C or W6Co6C. Thus, the thermal spray coating method has a disadvantage that can hardly obtain an excellent coating layer.
In order to solve the problems of the thermal spray coating method, a cold spray technique capable of coating powders at a low temperature has been developed. In the cold spray technique, powder particles having a size of about 1-50 μm are accelerated to a velocity of 300-1200 m/sec, which exceeds a threshold velocity at which a coating material can be coated on a substrate, by using high pressure gases, such as nitrogen, helium and air. The particles strike the target surface, the kinetic energy of the particles is transformed into plastic deformation of the particles, and a bond is formed between the particles and the target surface.
Since the cold spray technique coats the particles in solid state without melting them, it can solve somewhat the problems of the thermal spray coating method. In addition, since there is no residual tensile stress caused by solidification stress, a thick coating is possible. Therefore, the cold spray technique can be applied to “near net shaping” process.
The cold spray technique is disclosed in U.S. Pat. Nos. 6,365,222 B1, 6,491,208 B2, 6,139,913 and 6,283,386, and U.S. Pat. Pub. Nos. 2001/0042508 A1, 2002/0033135 A1, U.S. Pat. No. 6,502,767 B2, 2002/0073982 A1, 2002/0102360 A1, U.S. Pat. No. 6,444,259 B1, 2002/0182311 A1, 2002/0182313 A1, 2002/0182314 A1, etc.
U.S. Pat. No. 6,365,222 B1 discloses a process of repairing components using a cold spray technique, and U.S. Pat. No. 6,491,208 B2 discloses a process of repairing turbine blade. Also, U.S. Pat. Nos. 6,139,913 and 6,283,368 disclose a nozzle that can accelerate gas to high velocity in the range of 1000 m/sec or more. Those patents can be applied to powder particles having size of 50 μm or more. In addition, those patents discloses a cross-sectional area ratio of a main gas passage to an injection tube in a mixing chamber for mixing the accelerating gas and the coating particles.
U.S. Pat. Pub. Nos. 2001/0042508 A1 and 2002/0033135 A1 and U.S. Pat. No. 6,502,767 B2 disclose a method of easily disassembling a cold spray nozzle. A material for main feed tube and a maximum preheating temperature (700° C.) are described in those publications and patent.
U.S. Pat. Pub. No. 2002/0073933 A1 discloses a method of applying a cold spray in coating a cylinder inner wall of a car engine block.
U.S. Pat. Pub. No. 2002/0102360 A1 and U.S. Pat. No. 6,444,259 B1 disclose a thermal barrier coating and an applying method thereof.
U.S. Pat. Pub. Nos. 2002/0182311 A1, 2002/0182313 A1 and 2002/0182314 A1 disclose a method of manufacturing electric machines using kinetic spray.
The above-described cold spray techniques are useful in various application fields, but have problems to be solved.
First, there is a limit to usable materials because solid materials are used in the cold spray techniques. Specifically, ceramic is very difficult to use in the cold spray technique, while pure copper, nickel or aluminum is widely used because of its high ductility.
Second, even the widely used materials must be sprayed at high velocity of more than threshold velocity so as to obtain an excellent coating characteristic. Otherwise, the yield may be degraded due to a low deposition rate.
Cermet materials, such as WC—Co, have high abrasive wear resistance and thus are widely used for industry. However, since the cermet materials have bad coating characteristic by cold spray, they are mainly used in the thermal spray coating technique. That is, the cermet materials are difficult to use in the cold spray technique.
The increase in the velocity of the accelerating gas can be achieved by increasing pressure of a gas supply unit. However, this method requires a large amount of gas so as to increase the gas pressure. Consequently, a large amount of gas is used so that economic efficiency gets worse.
In order to solve that problem, the accelerating gas is generally heated to about 400-600° C. so as to increase the gas velocity without increasing the pressure of the gas supply unit in the cold spray apparatus. The method is effective in increasing the velocity of the accelerating gas because specific volume and pressure of gas can be increased and the adiabatic expansion effect at the nozzle can be obtained by this method.
However, if the method alone is used, it is difficult to obtain a satisfactory deposition rate, especially in the coating of cermet materials. Accordingly, the gas heater must heat the gas more higher so as to increase gas temperature, resulting in increase of the power consumption. In addition, a lifetime of a tube in the gas heater is shortened and thus there is a limit in the increase of temperature.